middleware/metrics: fix crash on startup (#318)

Make the methods that handle Metrics all use pointer receivers to fix
sync.Once not being initialized.

Finish the setup_test to test for failures. And make the check for the
address more strict and return an error when it does not have a port
number.

Add a toplevel test that starts a CoreDNS server with metrics enabled
so we catch these errors in the future.

middleware/metrics/metrics.go

@@ -32,7 +32,7 @@ type Metrics struct {
 	Addr      string
 	ln        net.Listener
 	mux       *http.ServeMux
-	Once      *sync.Once
+	Once      sync.Once
 	ZoneNames []string
 }